Commercial photo-film cases are in general packed with a transparent wrapping film, in lots of 10 or 20 unit cases or so, i.e., bundle-wrapped when freshly delivered from the factory.
Conventional package materials heretofore used are moisture-proof cellophanes having polyvinylidene chloride coatings on both surfaces thereof, and non-stretched low density polyethylenes. Those conventional materials have, however, various problems as mentioned below, and the improvements thereon have been desired. More specifically, moisture-proof cellophanes have the following defects:
(a) The contraction and expansion are great, depending upon the moisture. In a moist atmosphere of high humidity, therefore, the cellophanes absorb the moisture to elongate and curl, further resulting in the occurrence of blocking with packaged (wrapped) cases. In a dry atmosphere of low humidity, on the contrary, they will shrink and the sealed portions come to peel off or the wrapped cases are often depressed or crushed thereby.
(b) When exposed to ultraviolet rays or when dried and dehydrated, they become brittle and are often broken.
(c) The thermal contraction during the processing operation is poor, and therefore, mechanical tight wrapping is difficult and the package of unit cases bundle-wrapped in one lot will become loose and rickety.
(d) The impact strength is poor. In particular, the tear strength is poor in a less moist and dehydrated state, and they are often broken.
(e) The cost is high.
On the other hand, the non-stretched polyethylenes also have the following drawbacks:
(a) Heat-sealing is difficult. (This is because, when the temperature is elevated, other portions than the heat-sealed portions are melted.)
(b) The thermal contraction is poor and the wrapped package in lots will become rickety.
(c) The physical strength and rigidity are poor, and the film is required to be thick, which is disadvantageous with respect to the transparency and the cost thereof. Even if the film is made thick, the surface is apt to be scratched, and the film is likely to become opaque.
(d) The smoothness, gloss and moisture-proofness are poor.
With respect to the thermal contraction, among these problems, this is naturally improved and increased by using a stretched-film, which, however, results in occurrence of wrinkles during mechanical packaging operation, and this is of no practical use. Under the circumstances, various studies have been made on the use of low-stretched films in such degree that they do not wrinkle while processed. However, any simple technique has not heretofore been found, enough to satisfy the thermal contraction without occurrence of wrinkles.